The present invention relates to a method of controlling hydraulic pressures in a speed changing mechanism having a plurality of hydraulic clutches, that is, a hydraulic power shift speed change mechanism. Particularly, the invention relates to a method of controlling hydraulic pressures in a multistep-speed-type speed change mechanism constituted such that a plurality of hydraulic type speed change units are connected in tandem, wherein each of the hydraulic type speed change units is constituted of a plurality of transmission trains, and a hydraulic clutch is provided in each of the transmission trains.
Conventionally, there is publicly known a so-called hydraulic power shift speed change mechanism configured of a plurality of hydraulic clutches (fluid-operated multidisc clutches). Particularly, there is publicly known a multistep-speed-change-type speed change mechanism constituted such that a plurality of hydraulic type speed change units are connected in tandem, wherein each of the hydraulic type speed change units is constituted of a plurality of transmission trains, and a hydraulic clutch is provided in each of the transmission trains. In vehicles including the speed change mechanism, such as an agricultural and other work tractors, speed-changing for the number of steps obtained by multiplying the numbers of transmission trains provided in individual speed change units. Suppose a speed change mechanism configured of two hydraulic type speed change units, in which two transmission trains are provided in one of the hydraulic type speed change units, and three transmission trains are provided in the other hydraulic type speed change unit. In this case, 2xc3x973 steps are obtained; that is, totally, six-step speed changes can be performed.
Conventionally, to perform input/output control for engagement/disengagement operating fluid for individual hydraulic clutches in the above-described speed change mechanism, electromagnetic-type selector valves are used.
In connection with the conventional hydraulic-pressure control for the hydraulic clutches at the time of speed-changing, first of all, the relationship in time between engagement-objective clutches and disengagement-objective clutches will be described below. Essential things regarding speed-changing include the prevention from a case where double transmission trains are operated to be in transmission states. Specifically, in the above-described multistep-speed-change-type speed change mechanism configured by combining the plurality of hydraulic type speed change units, it is essential to avoid a case where two clutches are operated in an engaged state in each of the speed change units. Therefore, conventionally, a disengagement-objective clutch is first disengaged substantially completely; and after a nontransmission state is once made in the speed change mechanism, the engagement of the engagement-objective clutch is then started. However, a high load is imposed during a nontransmission state, the vehicle is likely stopped. In addition, since a hydraulic pressure begins to rise from the nontransmission state when the engagement-objective clutch starts engagement, there remain problems which cannot be solved in that great shocks occur, thereby causing an operator to feel uncomfortable.
In view of the above, as described below in the xe2x80x9cDisclosure of Inventionxe2x80x9d and in other portions, even when the transmission efficiency is reduced to the lowest level during speed-changing, at least either the disengagement-objective clutches or the engagement-objective clutches are controlled to be in slip states. Specifically, operating timing and a time-transitional hydraulic pressure property for the individual disengagement-objective clutch and the individual engagement-objective clutch are set so that a region representing a slip state (the region will hereinbelow be referred to as a xe2x80x9ccommon slip regionxe2x80x9d) for the two clutches can be secured.
Hereinbelow, a brief description will be made regarding clutch hydraulic pressures in the slip state. The pressure for a disengaged clutch in a fluid chamber is substantially 0, and a piston for operating a clutch disc is in a free state. To engage the disengaged clutch, first, fluid is fed to a fluid chamber therefor to be filled out, and the filled out fluid must be used to increase the pressure to hold the piston. When a hydraulic pressure having a value that is sufficiently high to hold at least the piston is set to a piston-holding pressure, the hydraulic piston is brought to a slip state at an operating hydraulic pressure that is higher than the piston-holding pressure.
However, different from the above-described conventional hydraulic-pressure control for which the relationship between the individual hydraulic pressure states for the disengagement-objective clutch and the engagement-objective clutch need not be taken into account, in the hydraulic-pressure control of the present invention, when the individual time-transitional hydraulic pressure properties for the engagement-objective clutch and the disengagement-objective clutch are fixed as have been set under specific conditions where, for example, the engine is operated at a rated revolution frequency, there occurs cases wherein no common slip region can be secured because of the conditional variations.
For example, in a speed change mechanism configured of two hydraulic type speed change units, there are two speed-changes. One of the speed changes is performed such that in one of the hydraulic type speed change units, clutches remain held in engaged states; and in the other hydraulic type speed change unit, one engaged clutch is disengaged, and a different clutch is newly engaged (one-objective-based hydraulic clutches are disengaged/engaged). The other speed change is performed such that, in each of the hydraulic type speed change units, one engaged clutch is disengaged, and a different clutch is newly engaged; that is, in the overall speed change mechanism, totally, two clutches are disengaged, and two clutches are engaged (two-objective-based hydraulic clutches are disengaged/engaged). As described above, before an engagement-objective clutch is controlled to be in a slip state, wait time is required until the pressure increases up to the level of the piston-holding pressure after the fluid is injected into the fluid chamber of the clutch. For two-objective-based hydraulic clutches to be disengaged/engaged, aforementioned time is required substantially twice as much as that in the case where one-objective-based hydraulic clutches are disengaged/engaged. Therefore, when clutch-timing and a time-transitional hydraulic pressure property are set to secure a common slip region according to the case where the one-objective-based hydraulic clutches are disengaged/engaged, they are not suitable to the case where the two-objective-based hydraulic clutches are disengaged/engaged.
When the engine revolution frequency is reduced, time required for filling out the fluid in the clutch fluid chamber is increased. Therefore, for example, hydraulic-pressure control is set to obtain a common slip region during a rated revolution. However, problems similar to the above can arise during idle revolution.
In comparison between a speed-changing operation at a shifting-up time and a speed-changing operation at a shifting-down time, in the former case, since the relative revolution speed of a secondary-side rotation shaft with respect to that on a primary side of an engaged/disengaged is increased, a common-slip-region period needs to be set to be relatively long. On the other hand, in the latter case, the speed-changing is performed to reduce the relative revolution speed of the same secondary-side rotation shaft, and rotational inertia at a time of preshift operation is imposed on the secondary-side rotation shaft. Therefore, the common-slip-region period may be short; and when it is long, smooth speed-changing is impaired.
As in the conventional case, in speed-changing in which an engagement-objective clutch is engaged after a disengagement-objective clutch is disengaged, detection is performed by using a pressure sensor or the like for the state of engagement of the disengagement-objective clutch that is supposed to have been engaged. Checking is thereby performed for abnormality (such as entrance of foreign substances). Thereafter, engagement of the engagement-objective clutch is interrupted, thereby allowing double transmission to be avoided. As in the case of the present invention, in the speed-changing in which a common slip region is secured, disengaging operations and engaging operations of clutches are overlapped. Therefore, there can be caused a case where a disengagement-objective clutch is not disengaged, while an engagement-objective clutch is engaged. That is, there can be caused double transmission that can cause damage in the transmission mechanism. Therefore, an abnormality-detecting method suitable to the present invention is demanded.
Pressure-increase properties required for the engagement-objective clutches at the time of speed-changing are different depending on the traveling mode of a work vehicle employing the speed change mechanism; that is, the properties differ depending on whether the vehicle is engaged in normal (on-the-road) traveling or tractional traveling. In a tractional travel time, the hydraulic pressure at a rising time needs to be set high, and the clutch needs to be quickly engaged. Otherwise, the transmission efficiency is not sufficient to catch up with the load, thereby causing engine failure. To reduce shock that can be caused in a normal travel time, rising pressure is preferably controlled as low as possible.
Conventionally, to overcome these problems, in a hydraulic-pressure control system for hydraulic clutches, two types of pressure-increase properties, one for normal traveling and another for tractional traveling so as to be alternatively selected by an operator are stored.
However, problems still remain pending resolution. With a control method that is dependent on operator""s switching operation, when erroneous operation is performed, there occurs hydraulic-pressure increase that does not correspond to practical requirements, causing problems such as engine failure and shock generation. To cope with these problems, the control is preferably arranged such that the load state is automatically can be detected, and one of the hydraulic-pressure-increase properties can be selected according to the result of the detection.
In addition, as described above, the variety of conditions varies the requirements regarding, for example, hydraulic-clutch engagement/disengagement operations at the time of speed-changing, i.e., the time-transitional hydraulic-pressure-increase properties for engagement-objective clutches, time-transitional hydraulic-pressure-decrease properties, and the operational timing. To comply with these requirements, it is preferable that input/output hydraulic pressures for clutches be controlled to be variable; that is, it is preferable that the capacity of an individual clutch-operating valve be variable.
The present invention relates to a speed change mechanism (so-called hydraulic power shift speed change mechanism) having a plurality of speed-changing hydraulic clutches, each of which is engaged according to hydraulic-pressure-increase effects and is disengaged according to hydraulic-pressure-decrease effects. A primary object of the invention is to avoid a nontransmission state that can occur at a time of speed-changing with the speed change mechanism.
To achieve the object, according to the present invention, at a time of speed-changing operation, an operating hydraulic pressure for a clutch to be engaged from a disengaged state is gradually increased in a time transition, and an operating hydraulic pressure for the clutch to be disengaged from an engaged state is reduced during the gradual pressure increase. Preferably, during the speed-changing operation, an operating-hydraulic-pressure-decrease start time for the disengagement-objective clutch is set to be later than an operating-hydraulic-pressure-increase start time at which a fluid chamber of the engagement-objective clutch becomes full of fluid, and the pressure thereof rises to a piston-holding pressure. Thereby, a time-transitional pressure region (common slip region) where an engagement-objective clutch and a disengagement-objective clutch commonly slip at the time of speed-changing operation is secured.
Also, in connection with the aforementioned object, in order to allow the common slip region to be constantly secured at all times regardless of various conditional variations, at least one of a time difference between the operating-hydraulic-pressure-increase start time for the engagement-objective clutch and the operating-hydraulic-pressure-decrease start time for the disengagement-objective clutch and a time-transitional decrease property in the operating pressure for the disengagement-objective clutch is controlled to vary corresponding to engine revolution frequencies.
In this case, the various conditions include engine revolution frequency. Corresponding to the property that a fluid-chamber filling-out time for the engagement-objective clutch increases in proportion to reduction in the engine revolution frequency, when the time difference is controlled to vary, the aforementioned time difference is set longer in proportion to reduction in the engine revolution frequency or in a case where the engine revolution frequency is equal to or lower than a specific level so as to decrease slower in proportion to reduction in the engine revolution frequency or in a case where the engine revolution frequency is equal to or lower than a specific level.
In the speed change mechanism (so-called multistep-speed-change-type speed change mechanism) configured by classifying the aforementioned plurality of speed-changing hydraulic clutches to allocate them to a plurality of hydraulic type speed change units, the hydraulic clutches are alternatively engaged in each of the hydraulic type speed change unit to thereby form one speed step. In this configuration, as described above, in order to secure the time-transitional pressure region (common slip region) where the engagement-objective clutch and the disengagement-objective clutch at the time of speed-changing commonly slip, when the hydraulic-pressure control in which the operating hydraulic pressure for the clutch to be engaged from a disengaged state is gradually increased in the time transition, and an operating hydraulic pressure for the clutch to be disengaged from an engaged state is reduced during the gradual pressure increase at the time of speed-changing is employed, the number of clutches to be engaged/disengaged is included as one of the aforementioned various conditions. Therefore, when the time difference is controlled to vary, the time difference is set relatively long at a time of speed-changing when the number of the clutches to be engaged/disengaged is large, and the time-transitional decrease property is controlled to vary, the time-transitional decrease property is set to be reduced slower at a time of speed-changing when the number of the clutches to be engaged/disengaged is large.
Considering that a rotational inertia is imposed at a time of shifting-down operation compared to a case at a time of the shifting-up operation, in order to reduce the area of a common slip region at the time of the shifting-down operation to be narrower than that at the time of shifting-up operation, at least one of a time difference between the operating-hydraulic-pressure-increase start time for the engagement-objective clutch and the operating-hydraulic-pressure-decrease start time for the disengagement-objective clutch and a time-transitional decrease property in the operating pressure for the disengagement-objective clutch is controlled to vary depending on whether the speed-changing operation is a shifting-up operation or a shifting-down operation. For example, the time difference is set to be relatively short.
In this case, it is preferable that, during speed-changing, regardless of variations in the time difference and the time-transitional decrease property that have been set to meet the aforementioned individual conditions, the operating-hydraulic-pressure-decrease start time for the disengagement-objective clutch be set to be later than the operating-hydraulic-pressure-increase start time at which the fluid chamber of the engagement-objective clutch becomes full of fluid, and the pressure thereof rises to the piston-holding pressure.
Another object of the present invention is to provide an appropriate method to detect an abnormal clutch to prevent the occurrence of a double-transmission state in the hydraulic power shift speed change mechanism for which the hydraulic-pressure control is performed as described above.
To achieve this object, a pressure-detecting means is provided to detect an operating hydraulic pressure for each of the hydraulic clutches, and when the number of the pressure-detecting means for detecting hydraulic pressures higher than a specific pressure value is greater than the number of the hydraulic clutches to be engaged at the time of speed-changing operation (in the speed change unit configured of the plurality of hydraulic type speed change units that are connected to in tandem, when two or more units of the detecting means each detect a pressure higher than a specific pressure value in at least in one of the hydraulic type speed change units), one of two hydraulic-pressure control operations is performed, one hydraulic-pressure control operation being performed to engage only those of the hydraulic clutches which have immediate-previously been disengaged, and the other one hydraulic-pressure control operation being performed to disengage all the hydraulic clutches.
The individual pressure-detecting means may be configured such that the individual means constitute switches each turning ON or OFF with respect to the border of the specific pressure value, and when the number of the pressure-detecting means for detecting hydraulic pressures higher than a specific pressure value is greater than the number of the hydraulic clutches to be engaged at the time of speed-changing operation (in the speed change unit configured of the plurality of hydraulic type speed change units that are connected to in tandem, when two or more units of the detecting means each detect a pressure higher than a specific pressure value in at least in one of the hydraulic type speed change units), one of two hydraulic-pressure control operations is performed, one hydraulic-pressure control operation being performed to engage only those of the hydraulic clutches which have immediate-previously been disengaged, and the other one hydraulic-pressure control operation being performed to disengage all the hydraulic clutches.
Still another object of the present invention is to detect whether a load is imposed on a vehicle by using appropriate detecting means, not by relying on operator-performing switch operations. This allows operating hydraulic pressures for the individual hydraulic clutches to be appropriately increased without failure.
To achieve this object, in the present invention tractional-load detecting means is provided in a vehicle employing the speed change mechanism to thereby modify a time-transitional increase property in the operating pressure for the hydraulic clutch to be engaged at the time of speed-changing and a time-transitional decrease property in the operating pressure for the hydraulic clutch to be disengaged at the time of speed-changing depending on whether or not the tractional-load detecting means detects a tractional load. Alternatively, when a governor mechanism capable of performing control of an engine revolution frequency according to detection of an engine load is provided in the vehicle employing the speed change mechanism, the governor is used to modify a time-transitional increase property in the operating pressure for the hydraulic clutch to be engaged at the time of speed-changing depending on whether or not the governor mechanism detects an engine load equal to or higher than a specific level.
The above load detection may be used to modify a time-transitional decrease property in the operating pressure for the hydraulic clutch to be disengaged at the time of speed-changing.
As summarized above, the speed change mechanism comprising hydraulic clutch according to the present invention, corresponding to the various conditions modifies the time difference between the operating-hydraulic-pressure-increase start time for the engagement-objective clutch, the operating-hydraulic-pressure-decrease start time for the disengagement-objective clutch, and the time-transitional decrease property in the operating pressure for the disengagement-objective clutch at the time of speed-changing operation. Therefore, in order to allow input/output pressures of operating fluid fed to each of the hydraulic clutches to be adjustable, the individual hydraulic clutch is controlled by means of an electromagnetic pressure proportion valve provided for each of the hydraulic clutches.
The above and other objects, configurations, and advantages of the invention will become apparent from the following detailed description thereof taken in conjugation with the accompanying drawings.